This is certainly interesting. He may have a sore throat or an inflammation of his stomach or oesophagus that causes him to regurgitate or vomit when excited or stimulated etc. Sometimes fur causes mechanical irritation and resulting inflammation that is responsible for this condition. Other conditions that your vet will need to rule is a foreign object lodged in his throat or in his oesophagus or stomach. He may also have to check for some viral diseases that may have been contracted from being outdoors.
Make sure you have him protected and on good parasite control. I guess a starting point to try and rule out what is going on is a fecal test to check for intestinal parasites and an x-ray to check for obstructions from foreign bodies incl. fur balls.
Last thought is that there may be a food intolerance causing an IBD (Inflammatory bowel disease) and this will be something that your vet may need to address also.
Martin Slome DVM
Centre Street Animal Hospital
7700 Bathurst Street Units 40-42
Tel. (905) 771-9855